Virus

Genetics
Copyright Genetics Society of America

Virus

A virus is a parasite that must infect a living cell to reproduce. Although viruses share several features with living organisms, such as the presence of genetic material (DNA or RNA), they are not considered to be alive. Unlike cells, which contain all the structures needed for growth and reproduction, viruses are composed of only an outer coat (capsid), the genome, and, in some cases, a few enzymes. Together these make up the virion , or virus particle. Many illnesses in humans, including AIDS, influenza, Ebola fever, the common cold, and certain cancers, are caused by viruses. Viruses also exist that infect animals, plants, bacteria, and fungi.

Physical Description and Classification

Viruses are distinguished from free-living microbes, such as bacteria and fungi, by their small size and relatively simple structures. Diminutive viruses such as parvovirus may have a diameter of only 25 nanometers (nm, 10-9 meters). Poxviruses, the largest known viruses, are about 300 nanometers across, just at the detection limits of the light microscope. Typical bacteria have diameters of 1,000 nanometers or more. Information on the structure of viruses has been obtained with several techniques, including electron

microscopy (EM). The limit of resolution of traditional EM is about 5 nm. With advanced EM techniques, such as cryogenic EM (cryoEM, in which the sample is rapidly frozen instead of exposed to chemical fixatives), coupled with computer image processing, smaller structures (1-2 nm) can be resolved. However, X-ray crystallography is the only method that allows for atomic-level resolution. Small viruses that produce uniform particles can be crystallized. The first atomic-level structure of a virus, tomato bushy stunt virus, was solved in 1978.

There is great diversity among viruses, but a limited number of basic designs. Capsids are structures that contain the viral genomes; many have icosahedral symmetry. An icosahedron is a three-dimensional, closed shape composed of twenty equilateral triangles. Viral proteins, in complexes termed "capsomers," form the surface of the icosahedron.

Other viruses, such as the virus that causes rabies, are helical (rod shaped). The length of helical viruses can depend on the length of the genome, the DNA or RNA within, since there are often regular structural interactions between the nucleic acids of the genome and the proteins that cover it.

A lipid -containing envelope is a common feature of animal viruses, but uncommon in plant viruses. Embedded in the envelope are surface proteins, usually glycoproteins that help the virus interact with the surface of the cell it is infecting. A matrix layer of proteins often forms a bridge between the surface glycoproteins and the capsid. Some viruses, such as the picornaviruses, are not enveloped, nor do they have a matrix layer. In these viruses, cell-surface interactions are mediated by the capsid proteins.

Some viruses have compound structures. The head of the T4 bacterial virus (bacteriophage) is icosahedral and is attached via a collar to a contractile

Molecule

Sequence

Polarity or Sense

Complementary RNA

A U U G G G C U C

negative

Coding strand DNA

T A A C C C G A G

positive

Complementary DNA

A T T G G G C T C

negative

mRNA

U A A C C C G A G

positive

tail with helical symmetry. Large viruses, such as the herpesviruses and poxviruses, can have higher-ordered and more complex structures.

Classification of viruses considers the genome characteristics, virion shape and macromolecular composition, and other properties, such as antigenicity and host range. A scheme for classification of viruses based on the type of nucleic acid (DNA or RNA) present in the virus particle and the method of genome replication was devised by David Baltimore, co-discoverer of reverse transcriptase (see Table 1). Reverse transcriptase is an enzyme that converts retroviral genomic single-stranded (ss) RNA into doubled-stranded (ds) DNA.

Viral genomes can be RNA or DNA, positive or negative in polarity, ss or ds, and one continuous (sometimes circular) molecule or divided into segments. By convention, messenger RNA (mRNA) that can be directly translated to protein is considered positive sense (or positive in polarity). DNA with a corresponding sequence (that is, the coding strand of double-stranded DNA) is also a positive-sense strand. An RNA or DNA molecule with the reverse complementary sequence to mRNA is a negative-sense strand. A few viruses have been identified that contain one or more "ambisense" genomic RNA segments that are positive sense in one part of the molecule (this part can be translated directly into protein) and negative sense (reverse complement of coding sequence) in the rest of the molecule.

Virus Replication Cycle

For a virus to multiply it must infect a living cell. All viruses employ a common set of steps in their replication cycle. These steps are: attachment, penetration, uncoating, replication, assembly, maturation, and release.

Attachment and Penetration.

A virion surface protein must bind to one or more components of the cell surface, the viral receptors. The presence or absence of receptors generally determines the type of cell in which a virus is able to replicate. This is called viral tropism. For example, the poliovirus receptor is present only on cells of higher primates and then in a limited subset of these, such as intestine and brain cells. While called virus receptors, these are actually used by the cell for its own purposes, but are exploited by the virus for entry.

Entry of the viral genome into the cell can occur by direct penetration of the virion at the cell surface or by a process called endocytosis, which is the engulfment of the particle into a membrane-based vesicle. If the latter, the virus is released when the vesicle is acidified inside the cell. Enveloped viruses may also fuse with the cellular surface membrane, which results in release of the capsid into the cytoplasm . Surface proteins of several viruses
contain "fusion peptides," which are capable of interacting with the lipid bilayers of the host cell.

Uncoating and Replication.

After penetration, viral capsid proteins must be removed, at least partly, to express and replicate the viral genome. In the case of most DNA viruses, the capsid is routed to the nucleus prior to uncoating. An example can be seen in the poxviruses, whose large DNA genomes encode most of the proteins needed for DNA replication. These viruses uncoat and replicate completely in the cytoplasm. RNA viruses typically lose the protective envelope and capsid proteins upon penetration into the cytoplasm. In reoviruses, only an outer protein shell is removed and replication takes place inside a structured subviral particle.

Viral genomes must be expressed as mRNAs in order to be translated into structural proteins for the capsids and, in some cases, as replicative proteins for replicating the virus genome. Viral genomes must also provide templates
that can be replicated to produce progeny genomes that will be packaged into newly produced virions. Replication details vary among the different types of viruses.

The ss positive-sense DNA of parvoviruses is copied by host DNA polymerase (the enzyme that replicates DNA) in the nucleus into a negative-sense DNA strand. This in turn serves as a template for mRNA and progeny DNA synthesis. The genomes of larger DNA viruses, with the exception of the poxviruses, are also transcribed and replicated in the nucleus by a combination of viral and host enzymes (for example, DNA-dependent RNA polymerses for transcription of mRNAs, DNA-dependent DNA polymerase for genome replication).

Positive-polarity RNA virus genomes can be translated directly, but for effective progeny production additional rounds of RNA replication via a negative-stranded intermediate are required. This is accomplished by a viral transcriptase (RNA-dependent RNA polymerase) and associated cofactors. Single-stranded negative-sense RNA viruses of animals must also carry a viral transcriptase to transcribe functional mRNAs and subsequently produce proteins, since this RNA-to-RNA enzymatic activity is typically lacking in animal cells.

Retroviruses are unique among viruses in that the genome is diploid, meaning that two copies of the positive-polarity RNA genome are in each virus particle. The genomic RNA is not translated into protein, but rather serves as a template for reverse transcription, which produces a double-stranded DNA via a viral reverse transcription enzyme. The DNA is subsequently integrated into the host cell chromosomal DNA. Hepadnaviruses also encode a reverse transcriptase, but replication occurs inside the virus particle producing the particle-associated genomic DNA.

Assembly, Maturation, and Release.

As viral proteins and nucleic acids accumulate in the cell, they begin a process of self-assembly. Viral self-assembly was first demonstrated in a seminal series of experiments in 1955, wherein infectious particles of tobacco mosaic virus spontaneously formed when purified coat protein and genomic RNA were mixed. Likewise, poliovirus capsomers are known to self-assemble to form a procapsid in the cytoplasm. Progeny positive-strand poliovirus RNAs then enter this nascent particle. "Chaperone" proteins (chaparonins) of the cell play a critical role in facilitating the assembly of some viruses. Their normal role is to help fold cellular proteins after synthesis.

The maturation and release stages of the replication cycle may occur simultaneously with the previous step, or may follow in either order. Many viruses assemble their various components into "immature" particles. Further intracellular or extracellular processing is required to produce a mature infectious particle. This may involve cleavage of precursors to the structural proteins, as in the case of retroviruses.

Viruses that are not enveloped usually depend upon disintegration or lysis of the cell for release. Enveloped viruses can be released from the cell by the process of budding. In this process the viral capsid and usually a matrix layer are directed to a modified patch of cellular membrane. Interactions between the matrix proteins and/or envelope proteins drive envelopment. In the case of viruses that bud at the cell surface, such as some
togaviruses and retroviruses (including HIV), this also results in release of the virus particles. If the virus acquires a patch of the nuclear membrane (as is the case with herpesviruses), then additional steps involving vesicular transport may be required for the virus to exit the cell.

Infection Outcomes

Viral infection can result in several different outcomes for the virus and the cell. Productive infection, such that each of the seven steps outlined above occurs, results in the formation of progeny viruses. Cells productively infected with poliovirus can yield up to 100,000 progeny virions per cell, although only a small fraction (fewer than 1 per 1,000) of these are capable of going on to carry out a complete replication cycle of their own. Productive infection may induce cell lysis, which results in the death of the cell. Nonenveloped viruses typically induce cell lysis to permit release of progeny virions. Many enveloped viruses also initiate events that result in cell death by various means, including apoptosis , necrosis , or lysis.

Viral infection may be abortive, in which one or more necessary factors, either viral or cellular, are absent and progeny virions are not made. Infection may be nonproductive, at least transiently, but viral genomes may still become resident in the host cell. Herpesviruses and retroviruses can establish latent infections. Latently infected cells may express a limited number of viral products, including those that result in cell transformation. Latent infections can often be activated by various stimuli, such as stress in the case of herpesviruses, to undergo a productive infection.

Viral Cancers

Infection with certain viruses can also result in cell transformation, stable genetic changes in the cell that result in disregulated cell growth and extended growth potential (immortalization). In animals, such virally induced cellular changes can result in cancer. This correlation was first made by Harry Rubin and Howard Temin in the 1950s, when they observed that Rous sarcoma virus, a retrovirus capable of inducing solid tumors in chickens, could also cause biochemical and structural changes and extend the proliferative potential of cultured chicken cells.

Viruses are perhaps second only to tobacco as risk factors for human cancers. DNA tumor viruses include papillomaviruses and various herpes viruses (such as HHV-8, which causes Kaposi's sarcoma ). More than sixty strains of human papillomaviruses (HPV) have been identified. HPV cause warts, which are benign tumors, but are also the causes of malignant penile, vulval, and cervical cancers. Infection with hepatitis B or C viruses is associated with increased incidence of liver cancer. Adenoviruses have been shown to induce cancers in animals, but not in humans. Retroviruses can also cause cancer in various animal species, including humans. HTLV-1 causes adult T-cell leukemia in about 1 percent of infected humans.

Viruses can cause cancer through their effects on two important cellular genes or gene products: tumor suppressors and oncogenes . These genes are critical players in cell-cycle regulation. One protein product from HPV binds to the retinoblastoma (Rb) tumor suppressor protein. HPV E6 protein binds p53 tumor suppressor protein and promotes its degradation.
Acutely transforming retroviruses, which induce tumors in a short time period of weeks to months, carry modified versions of cellular oncogenes, called viral oncogenes. Slowly transforming retroviruses also subvert cellular oncogenes, but by integrating into or near the oncogene, thereby altering its expression, a process that can take years because of the apparently random nature of retrovirus integration.

Vaccines

Many viral infections can be prevented by vaccination. Several classes of vaccines are currently in use in humans and animals. Inactivated vaccines, such as the poliovirus vaccine developed by Jonas Salk, are produced from virulent viruses that are subjected to chemical treatments that result in loss of infectivity without complete loss of antigenicity (antigenicity is the ability to produce immunity). Another approach is the use of weakened variants of a virus with reduced pathogenicity to induce a protective immune response
without disease. While vaccines are usually given before exposure to a virus, postexposure vaccines can cure some virus infections with extended incubation periods, such as rabies.

Vaccines against smallpox eradicated the illness in 1980. It is believed that it may also be possible to eliminate polio. A recombinant vaccine against hepatitis B virus is now produced in yeast. However, developing effective vaccines to some viruses, including the common cold viruses, HIV-1, herpesviruses, and HPV, is proving very difficult principally due to the existence of many variants. Public health measures, such as mosquito control programs to curb the spread of viral diseases transmitted by these vectors , and safe-sex campaigns to slow the spread of sexually transmitted diseases, can also be effective. Because viruses replicate in cells, drugs that target viruses typically also affect cell functions. These therapeutic agents must be active against the virus while having "acceptable toxicity" to the host organism. The majority of the specific antiviral drugs currently in use target viral enzymes. For example, nucleoside analogues that target viral polymerases are active against HIV and certain herpesviruses.

Virus

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Virus

A virus is a small, infectious agent that is made up of a core of genetic material surrounded by a shell of protein. The genetic material (which is responsible for carrying forward hereditary traits from parent cells to offspring) may be either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Viruses are at the borderline between living and nonliving matter. When they infect a host cell, they are able to carry on many life functions, such as metabolism and reproduction. But outside a host cell, they are as inactive as a grain of sand.

Viruses cause disease by infecting a host cell and taking over its biochemical functions. In order to produce new copies of itself, a virus must use the host cell's reproductive "machinery." The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the organism.

Viruses can infect plants, bacteria, and animals. The tobacco mosaic virus, one of the most studied of all viruses, infects tobacco plants. Animal viruses cause a variety of diseases, including AIDS (acquired immuno deficiency syndrome), hepatitis, chicken pox, smallpox, polio, measles, rabies, the common cold, and some forms of cancer.

Viruses that affect bacteria are called bacteriophages, or simply phages (pronounced FAY-jez). Phages are of special importance because

Words to Know

Adult T cell leukemia (ATL): A form of cancer caused by the retro-virus HTLV.

AIDS (acquired immunodeficiency syndrome): A set of life-threatening, opportunistic infections that strike people who are infected with the retrovirus HIV.

Gene: Unit of heredity contained in the nucleus of cells that is composed of DNA and that carries information for a specific trait.

Host cell: The specific cell that a virus targets and infects.

HIV (human immunodeficiency virus): The retrovirus that causes AIDS.

Human T cell leukemia virus (HTLV): The retrovirus that causes ATL.

Infectious: Relating to a disease that is spread primarily through contact with someone who already has the disease.

Lysogenic cycle: A viral replication cycle in which the virus does not destroy the host cell but coexists within it.

Lytic cycle: A viral replication cycle in which the virus destroys the host cell.

Metabolism: The sum of all the physiological processes by which an organism maintains life.

Orthomyxovirus: Group of viruses that causes influenza in humans and animals.

Proteins: Complex chemical compounds that are essential to the structure and functioning of all living cells.

Retrovirus: A type of virus that contains a pair of single stranded RNA molecules joined to each other.

Reverse transcriptase: An enzyme that makes it possible for a retro-virus to produce DNA from RNA.

Ribonucleic acid (RNA): Genetic material consisting of a single strand of nucleic acid.

they have been studied much more thoroughly than have viruses. In fact, much of what we now know about viruses is based on the study of phages. Although there are both structural and functional differences between the two, they share many characteristics in common.

Structure of viruses

Although viral structure varies considerably among different types of viruses, all viruses share some common characteristics. All viruses contain either RNA or DNA surrounded by a protective protein shell called a capsid. The genetic material in a virus may take one of four forms: a double strand of DNA, a single strand of DNA, a double strand of RNA, or a single strand of RNA. The size of the genetic material of viruses is often quite small. Compared to the 100,000 genes that exist within human DNA, viral genes number from 10 to about 200 genes.

Viruses exist in one of three forms, as shown in Figure 1. They are named on the basis of their general shape as rodlike, icosahedral (having 20 sides), or spherical. Some viruses also have an outer covering known as an envelope that surrounds the capsid. The outer surface of some kinds of viral particles contain threadlike "spikes" that are often used in helping a virus invade a host cell (for example, the spherical virus in Figure 1).

Viral infection

A virus remains totally inactive until it attaches itself to and infects a host cell. Once that happens, the virus may follow one of two paths. First, the virus may insert its genetic material (it is always DNA in this case) into the DNA of the host cell. The combined host-viral DNA is then

carried along in the host cell as it lives and reproduces, generation after generation. Viruses that follow this pathway are said to be temperate or lysogenic viruses. At some point in the host cell's life, the viral DNA may be extracted (taken out) from the host DNA and follow the second pathway.

The second pathway available to viruses is called the lytic cycle. In the lytic cycle, the virus first attaches itself to the surface of the host cell. It then makes a hole in the cell membrane and injects its genetic material (DNA or RNA). The viral capsid is left behind outside the cell.

The next step depends on the nature of the viral genetic material, whether that material is single stranded or double stranded DNA or RNA. The end result of any one of the processes is that many additional copies of the viral capsid and the viral genetic material are made. These capsids and genetic material are then assembled into new viral particles. The single collection of genetic material originally injected into the host cell has been used to make dozens or hundreds of new viral particles.

When these particles have been assembled, they burst through the cell membrane. In the act, the host cell is destroyed. The new viral particles are then free to find other host cells and to repeat the process.

Retroviruses

Retroviruses make up an unusual group of viruses. Their genetic material consists of two single strands of RNA linked to each other. Retroviruses also contain an essential enzyme known as reverse transcriptase.

The unusual character of retroviruses is that they have evolved a method for manufacturing protein beginning with RNA. In nearly all living organisms, the pattern by which protein is manufactured is as follows: DNA in the cell's nucleus carries directions for the production of new protein. The coded message in DNA molecules is copied into RNA molecules. These RNA molecules then direct the manufacture of new protein. In retroviruses, that process is reversed: viral RNA is used to make new viral DNA. The viral DNA is then incorporated into host cell DNA, where it is used to direct the manufacture of new viral protein.

The first retrovirus discovered was the Rous sarcoma virus (RSV) that infects chickens. It was named after its discoverer, the American pathologist Peyton Rous (1879–1970). Other animal retroviruses are the simian immunodeficiency virus (SIV), which attacks monkeys, and the feline leukemia virus (FELV), which causes feline leukemia in cats. The first human retrovirus was discovered in 1980 by a research team headed by American virologist Robert Gallo (1937– ). Called human T cell leukemia virus (HTLV), this virus causes a form of leukemia (cancer of the blood) called adult T cell leukemia. In 1983–84, another human retro-virus was discovered. This virus, the human immunodeficiency virus (HIV), is responsible for AIDS.

The common cold and influenza

Two of the most common viral diseases known to humans are the common cold and influenza. The common cold, also called acute coryza or upper respiratory infection, is caused by any one of some 200 different viruses, including rhinoviruses, adenoviruses, influenza viruses, para-influenza viruses, syncytial viruses, echoviruses, and coxsackie viruses. Each virus has its own characteristics, including its favored method of transmission and its own gestation (developmental) period. All have been implicated as the agent that causes the runny nose, cough, sore throat, and sneezing that advertise the presence of the common cold. According to experts, more than a half billion colds strike Americans every year, an average of two infections for each man, woman, and child in the United States. In spite of intense efforts on the part of researchers, there are no cures, no preventative treatments, and very few treatments for the common cure.

Viruses that cause the common cold can be transmitted from one person to another by sneezing on the person, shaking hands, or handling an object previously touched by the infected person. Oddly, direct contact with an infected person, as in kissing, is not an efficient way for the virus to spread. In only about 10 percent of contacts between an infected and uninfected person does the latter get the virus.

Contrary to general opinion, walking around in a cold rain will not necessarily cause a cold. Viruses like warm, moist surroundings, so they thrive indoors in the winter. However, being outdoors in cold weather can dehydrate the mucous membranes in the nose and make them more susceptible to infection by a rhinovirus. The viruses that cause colds mutate with regularity. Each time a virus is passed from one person to the next, it may change slightly, so it may not be the virus the first person had.

The common cold differs in several ways from influenza, commonly known as the flu. Cold symptoms develop gradually and are relatively mild. The flu has a sudden onset and has more serious symptoms that usually put the sufferer to bed. The flu lasts about twice as long as the cold. Also, influenza can be fatal, especially to elderly persons. Finally, the number of influenza viruses is more limited than the number of cold viruses, and vaccines are available against certain types of flu.

Influenza. Influenza is a highly contagious illness caused by a group of viruses called the orthomyxoviruses. Infection with these viruses leads to an illness usually characterized by fever, muscle aches, fatigue (tiredness), and upper respiratory obstruction and inflammation. Children and young adults usually recover from influenza within 3 to 7 days with no complications. However, influenza can be a very serious disease among older adults, especially those over 65 with preexisting conditions such as heart disease or lung illnesses. Most hospitalizations and deaths from influenza occur in this age group. Although an influenza vaccine is available,

it does not offer complete protection against the disease. The vaccine has been shown only to limit the complications that may occur due to influenza.

Three types of orthomyxoviruses cause illness in humans and animals: types A, B, and C. Type A causes epidemic influenza, in which large numbers of people become infected during a short period of time. Flu epidemics caused by Type A orthomyxoviruses include the worldwide outbreaks of 1918, 1957, 1968, and 1977. Type A viruses infect both humans and animals and usually originate in Asia, where a large population of ducks and swine incubate the virus and pass it to humans. (Incubate means to provide a suitable environment for growth, in this case within the animals' bodies.) Asia also has a very large human population that provides a fertile ground for viral replication.

Type B influenza viruses are not as common as type A viruses. Type B viruses cause outbreaks of influenza about every two to four years. Type C viruses are the least common type of influenza virus and cause irregular and milder infections.

An important characteristic of all three kinds of influenza viruses is that they frequently mutate. Because they contain only a small amount of genetic material, flu viruses mutate frequently. The result of this frequent mutation is that each flu virus is different, and people who have become immune to one flu virus are not immune to other flu viruses. The ability to mutate frequently, therefore, allows these viruses to cause frequent outbreaks.

The most common complication of influenza is pneumonia, a disease of the lungs. Pneumonia may be viral or bacterial. The viral form of pneumonia that occurs with influenza can be very severe. This form of pneumonia has a high mortality rate. Bacterial pneumonia may develop when bacteria accumulate in the lungs. This type of pneumonia occurs five to ten days after onset of the flu. Because it is bacterial in origin, it can be treated with antibiotics.

Flu is treated with rest and fluids. Maintaining a high fluid intake is important, because fluids increase the flow of respiratory secretions, which may prevent pneumonia. A new antiviral medication is prescribed for people who have initial symptoms of the flu and who are at high risk for complications. This medication does not prevent the illness, but reduces its duration and severity.

A flu vaccine is available that is formulated each year against the current type and strain of flu virus. The vaccine would be most effective in reducing attack rates if it were effective in preventing influenza in schoolchildren. However, in vaccine trials, the vaccine has not been shown to be effective in flu prevention in this age group. In certain populations, particularly the elderly, the vaccine is effective in preventing serious complications of influenza and thus lowers mortality.

Virus

Encyclopedia of Small Business
COPYRIGHT 2007 Thomson Gale

Virus

A virus is a program designed to infect and potentially damage files on a computer that receives it. The code for a virus is hidden within a file or program—such as a text document or a spreadsheet program—and when the file is opened or the program is launched, the virus inserts copies of itself, infecting the computer on which these files are opened. Because of this ability to reproduce itself, a virus can quickly spread to other programs, including the computer's operating system. A virus may reside on a computer system for some time before taking any action detectable to the user. Other viruses may cause trouble immediately. Some viruses cause little or no damage. For example, a virus may manifest itself as nothing more than a message that appears on the screen at certain intervals. Other viruses are much more destructive and can result in lost or corrupted files and data. Viruses may render a computer unusable, necessitating the reinstallation of the operating system and applications. Viruses can even be written to imbed some small miscalculation into, for example, a spreadsheet program. This sort of hidden problem may jeopardize the accuracy of all the work done with the infected program for a long time before it is even detected.

Viruses are written to target program files and macros, or a computer's boot sector, which is the portion of the hard drive that executes the steps necessary to start the hardware and software. Program viruses attach themselves to the executable files associated with software programs, and can then attack any file that is used to launch an application, usually files ending with the "exe" or "com" extensions. Macro viruses infect program templates that are used to create documents or spreadsheets. Once infected, every document or spreadsheet opened with the infected program becomes corrupted. Boot sector viruses attack the computer's hard drive and launch themselves each time the user boots, or starts, the computer. Viruses are often classified as Trojan Horses or Worms. A Trojan Horse virus is one that appears harmless on the surface but, in reality, destroys files or programs. A Worm attacks the computer's operating system and replicates itself again and again, until the system eventually crashes.

Another line of viruses is referred to as Malware, or, more generically as Spyware. These are viruses that are imbedded in files downloaded onto an unsuspecting computer while the user is browsing the Internet. Spyware programs, once on a computer, allow the creator of the virus to snoop on or monitor the infected computer's browser activities. A spyware virus usually implants "pop-up" ads that will appear on a user's screen periodically. These programs can down a computer, cause it to crash, and in some cases can even record for the sender the recipient's credit card numbers if it is used to purchase items while online. Spyware is usually a nuisance-level virus but in some cases can pose a more serious threat.

The Internet, with its global reach and rapid delivery times, provides the ideal breeding ground for viruses. Typically, someone who wants to spread a virus does so by sending out an e-mail message containing an infected attachment. The subject line on such a message sounds innocuous, so unsuspecting recipients open the message, unwittingly infecting their computers. More insidious yet, many viruses infect the recipient and then launch e-mail messages using the recipient's e-mail system address book and send themselves out to all of the recipient's list of colleagues, clients, vendors, friends, and family for whom an e-mail is found.

VIRUS PROTECTION

With new computer viruses appearing daily, keeping a computer or network of computers free of viruses is a daunting task. If, however, one views the proper use and maintenance of anti-virus software as a necessary part of running computers, the task becomes just another in the list of things one must do to maintain computers. The following are steps every computer user should follow to protect his or her computer from viruses.

Install an anti-virus software program to identify and remove viruses before they can cause any damage. These programs scan, or review, files that may come from floppy diskettes, the Internet, e-mail attachments, or networks, looking for patterns of code that match patterns in the anti-virus software vendor's database of known viruses. Once detected, the software isolates and removes the virus before it can be activated.

Update the anti-virus software weekly. Because the number of viruses is increasing all the time, it is important to keep anti-virus software up-to-date with information on newly identified viruses. Anti-virus software vendors are constantly updating their databases of information on viruses and making this information available to their customers via their Web sites or by e-mail.

Maintain a regular back-up procedure for all computers. This procedure may be as simple as keeping copies of important files on diskettes or CD-ROMs (in which case original software should be kept with these diskettes or CDs) and it may be as elaborate as a system designed to produce a mirror copy of a system, updating this copy every few minutes. For a small business, the most prudent level of back-up probably falls somewhere between these two extremes. Whatever the schedule is, it should include regular and periodic backing up of all computer systems and the remote storage of the backups' media in case of fire, flood, etc.

Do not open e-mail from unknown recipients or messages that contain unexpected attachments. A user should delete these types of messages. As a general rule, a user should scan every e-mail attachment for viruses before opening it—even an expected attachment—as the sender may have unknowingly sent an infected file.

NOTEWORTHY VIRUSES

One of the most costly and memorable viruses was the Love Bug virus of 2000. This virus targeted users of Microsoft's Outlook e-mail program. Originating in the Philippines, the subject line on the Love Bug message was the inviting "ILOVEYOU." If a user opened the attachment to this message, the virus quickly began to destroy files, targeting digital pictures and music files. The Love Bug virus also perpetuated itself by forwarding the original message to all e-mail addresses listed in the current recipient's Outlook address book. In this way, the virus was able to circle the globe in just two hours. The virus brought businesses to a standstill as companies, large and small, were forced to shut off incoming Internet e-mail messages and repair infected systems. In all, the Love Bug virus is estimated to have cost up to $10 billion in lost work hours.

Another record-setting virus, the fastest spreading e-mail worm ever, is MyDoom. This ominously named virus is a computer worm affecting Microsoft Windows. It was first sighted on January 2004 and was designed to send junk e-mail through infected computers. Early speculation about MyDoom held that the sole purpose of the worm was to perpetrate a distributed "denial-of-service attack" against the SCO Group. A denial of service attack is one in which a company Web site is flooded with e-mail causing it to overload, or shut down, and damaging any sales generated through the site, or services provided on the site.

Most viruses do not reach the level of fame that these two achieved, either because they are programmed to change as they spread, making them harder to identify and stop, or because they attack a niche sector of the computer-using market. Nonetheless, the damage that such viruses, even seemingly innocuous ones, can have on a company are great. Lost computer processing power equals slower-functioning computers, a lost of time and productivity. Lost files take time to retrieve from backup systems, assuming good backups exist. And rebuilding a computer that has been damaged by a virus is time-consuming for both the technician and the user.

Protection is the best way to save time, money, and the wear and tear that computer problems can have on all the people involved.

virus

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

virus, parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes. In one stage of their life cycle, in which they are free and infectious, virus particles do not carry out the functions of living cells, such as respiration and growth; in the other stage, however, viruses enter living plant, animal, or bacterial cells and make use of the host cell's chemical energy and its protein- and nucleic acid–synthesizing ability to replicate themselves.

The existence of submicroscopic infectious agents was suspected by the end of the 19th cent.; in 1892 the Russian botanist Dimitri Iwanowski showed that the sap from tobacco plants infected with mosaic disease, even after being passed through a porcelain filter known to retain all bacteria, contained an agent that could infect other tobacco plants. In 1900 a similarly filterable agent was reported for foot-and-mouth disease of cattle. In 1935 the American virologist W. M. Stanley crystallized tobacco mosaic virus; for that work Stanley shared the 1946 Nobel Prize in Chemistry with J. H. Northrup and J. B. Summer. Later studies of virus crystals established that the crystals were composed of individual virus particles, or virions. By the early 21st cent. the understanding of viruses had grown to the point where scientists synthesized (2002) a strain of poliovirus using their knowledge of that virus's genetic code and chemical components required.

Viral Structure

Typically the protein coat, or capsid, of an individual virus particle, or virion, is composed of multiple copies of one or several types of protein subunits, or capsomeres. Some viruses contain enzymes, and some have an outer membranous envelope. Many viruses have striking geometrically regular shapes, with helical structure as in tobacco mosaic virus, polyhedral (often icosahedral) symmetry as in herpes virus, or more complex mixtures of arrangements as in large viruses, such as the pox viruses and the larger bacterial viruses, or bacteriophages. Certain viruses, such as bacteriophages, have complex protein tails. The inner viral genetic material—the nucleic acid—may be double stranded, with two complementary strands, or single stranded; it may be deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The nucleic acid specifies information for the synthesis of from a few to 50 different proteins, depending on the type of virus.

Viral Infection of a Host Cell

A free virus particle may be thought of as a packaging device by which viral genetic material can be introduced into appropriate host cells, which the virus can recognize by means of proteins on its outermost surface. A bacterial virus infects the cell by attaching fibers of its protein tail to a specific receptor site on the bacterial cell wall and then injecting the nucleic acid into the host, leaving the empty capsid outside. In viruses with a membrane envelope the nucleocapsid (capsid plus nucleic acid) enters the cell cytoplasm by a process in which the viral envelope merges with a host cell membrane, often the membrane delimiting an endocytic structure (see endocytosis) in which the virus has been engulfed.

Within the cell the virus nucleic acid uses the host machinery to make copies of the viral nucleic acid as well as enzymes needed by the virus and coats and enveloping proteins, the coat proteins of the virus. The details of the process by which the information in viral nucleic acid is expressed and the sites in the cell where the virus locates vary according to the type of nucleic acid the virus contains and other viral features. As viral components are formed within a host cell, virions are created by a self-assembly process; that is, capsomere subunits spontaneously assemble into a protein coat around the nucleic core. Release of virus particles from the host may occur by lysis of the host cell, as in bacteria, or by budding from the host cell's surface that provides the envelope of membrane-enveloped forms.

Some viruses do not kill host cells but rather persist within them in one form or another. For example, certain of the viruses that can transform cells into a cancerous state (see cancer) are retroviruses; their genetic material is RNA but they carry an enzyme that can copy the RNA's information into DNA molecules, which then can integrate into the genetic apparatus of the host cell and reside there, generating corresponding products via host cell machinery (see also retrovirus). Similarly, in bacterial DNA viruses known as temperate phages, the viral nucleic acid becomes integrated into the host cell chromosomal material, a condition known as lysogeny; lysogenic phages are similar in many ways to genetic particles in bacterial cells called episomes (see recombination).

Viral Diseases

Some human diseases are apparently caused by the body's response to virus infection: immune reaction to altered virus-infected cells, release by infected cells of inflammatory substances, or circulation in the body of virus-antibody complexes are all virus-caused immunological disorders. Viruses cause many diseases of economically important animals and plants, some transmitted by carriers such as insects. A retrovirus (HIV) causes AIDS, several viruses (e.g. Epstein-Barr virus, human papillomavirus) cause particular forms of cancer in humans, and many have been shown to cause tumors in animals. Other viruses that infect humans cause measles, mumps, smallpox, yellow fever, rabies, poliomyelitis, influenza, and the common cold.

The techniques of molecular biology and genetic engineering have made possible the development of antiviral drugs effective against a variety of viral infections. Viruses, like bacterial infective agents, act as antigens in the body and elicit the formation of antibodies in an infected individual (see immunity). Indeed, vaccines against viral diseases such as smallpox were developed before the causative agents were known. Some viruses stimulate cellular production of interferon, which inhibits viral growth within the infected cell.

Classification

Viruses are not usually classified into conventional taxonomic groups but are usually grouped according to such properties as size, the type of nucleic acid they contain, the structure of the capsid and the number of protein subunits in it, host species, and immunological characteristics.

See C. Zimmer, A Planet of Viruses (2011).

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Virus

Biology
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Virus

Viruses are not cells and are metabolically inert outside of living cells. They can infect organisms consisting of just one cell, such as a single bacterial cell, or the individual cells of multicellular organisms such as humans. They are small compared to the cells they infect and as such must live as intracellular parasites . They absolutely require cells to reproduce. Within the appropriate cell, viruses are able to program the cell to replicate themselves by hijacking the normal cellular systems. The extracellular form of a virus, also known as a virion, is stable enough to survive the conditions required for transmission from one cell to another. The virion is composed of a set of genes (encoded by ribonucleic acid [RNA] or deoxyribonucleic acid [DNA]), which is protected by a protein -containing coat. The coat is often characterized by regularity and symmetry in its structure and is capable of binding to and invading cells. On invasion of a susceptible cell the virion is disassembled to release the viral genome . Once the viral genome is released, viral genes are expressed to reprogram the biosynthetic activities of the cell so that large numbers of progeny virions may be produced by the cell. These virions are then released by the infected cell to invade other cells so that the process can be repeated.

virus

virus A particle that is too small to be seen with a light microscope or to be trapped by filters but is capable of independent metabolism and reproduction within a living cell. Outside its host cell a virus is completely inert. A mature virus (a virion) ranges in size from 20 to 400 nm in diameter. It consists of a core of nucleic acid (DNA or RNA) surrounded by a protein coat (see capsid). Some (the enveloped viruses) bear an outer envelope consisting of proteins and lipids. Inside its host cell the virus initiates the synthesis of viral proteins and undergoes replication. The new virions are released when the host cell disintegrates. Viruses are parasites of animals, plants, and some bacteria (see bacteriophage). Viral diseases of animals include the common cold, influenza, AIDS, herpes, hepatitis, polio, and rabies (see adenovirus; arbovirus; herpesvirus; HIV; myxovirus; papovavirus; picornavirus; poxvirus); some viruses are also implicated in the development of cancer (see retrovirus). Plant viral diseases include various forms of yellowing and blistering of leaves and stems (see tobacco mosaic virus). Antiviral drugs are effective against certain viral diseases and vaccines (if available) provide protection against others. See also interferon.

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virus

virus Submicroscopic infectious organism. Viruses vary in size from ten to 300 nanometres, and contain only genetic material in the form of DNA or RNA. Viruses are incapable of independent existence: they can grow and reproduce only when they enter another cell, such as a bacterium or animal cell, because they lack energy-producing and protein-synthesizing functions. When they enter a cell, viruses subvert the host's metabolism so that viral reproduction is favoured. Control of viruses is difficult because harsh measures are required to kill them. The animal body has, however, evolved some protective measures, such as production of interferon and of antibodies directed against specific viruses. Where the specific agent can be isolated, vaccines can be developed, but some viruses change so rapidly that vaccines become ineffective. See also antibody; immune system

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virus

virus A type of non-cellular ‘organism’ which has no metabolism of its own. It consists mainly or solely of a nucleic acidgenome (RNA or DNA) enclosed by protein; in some cases there is also a lipoprotein envelope. In order to replicate (multiply), a virus must infect a cell of a suitable host organism where it redirects the host-cell metabolism to manufacture more virus particles. The progeny viruses are released, with or without concomitant destruction of the host cell, and then can infect other cells. All types of organism, from bacteria to humans, are susceptible to infection by viruses; virus infections may be asymptomatic or may lead to more or less severe disease.

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virus

vi·rus / ˈvīrəs/ • n. an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host: [as adj.] a virus infection. ∎ inf. an infection or disease caused by such an agent. ∎ fig. a harmful or corrupting influence: the virus of cruelty that is latent in all human beings. ∎ (also com·pu·ter vi·rus) a piece of code that is capable of copying itself and typically has a detrimental effect, such as corrupting the system or destroying data.

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virus

virus A type of non-cellular ‘organism’ which has no metabolism of its own. It consists mainly or solely of a nucleic acid genome (RNA or DNA) enclosed by protein; in some cases there is also a lipoprotein envelope. In order to replicate (multiply), a virus must infect a cell of a suitable host organism, where it redirects the host-cell metabolism to manufacture more virus particles. The progeny viruses are released, with or without concomitant destruction of the host cell, and then can infect other cells. All types of organism are susceptible to infection by viruses; virus infections may be asymptomatic or may lead to more or less severe disease.

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virus

virus A type of non-cellular ‘organism’ which has no metabolism of its own. It consists mainly or solely of a nucleic acid genome (RNA or DNA) enclosed by protein; in some cases there is also a lipoprotein envelope. In order to replicate (multiply), a virus must infect a cell of a suitable host organism where it redirects the host-cell metabolism to manufacture more virus particles. The progeny viruses are released, with or without concomitant destruction of the host cell, and then can infect other cells. All types of organism are susceptible to infection by viruses; virus infections may be asymptomatic or may lead to more or less severe disease.

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virus

virus From the Latin for toxin. These are very tiny microorganisms not visible with an ordinary microscope but only through electron microscopy. They have a very simple structure, and often consist only of DNA or RNA with a protein coat. They therefore lack some of the chemicals and enzymes needed for replication, so instead they infect other living cells and take over the infected cell's internal ‘machinery’.

virus

virus (vy-rŭs) n. a minute particle that is capable of replication but only within living cells. Viruses are too small to be visible with a light microscope or to be trapped by filters. They infect animals, plants, and microorganisms. Viruses cause many diseases, including herpes, influenza, mumps, polio, AIDS, and rabies. Antiviral drugs are effective against some of them, and many viral diseases are controlled by means of vaccines. —viral adj.

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